Rotator cuff tendon pathology is an important source of musculoskeletal morbidity, especially in athletic and working populations. Epidemiologic studies have linked repetitive physical exertions to rotator cuff disorders. Anatomic observations have demonstrated a hypovascular region of the supraspinatus tendon near its insertion. It has also been demonstrated that tension applied to tendon, which occurs during physical exertions, impairs the blood flow within tendon. Therefore, it has been speculated that local hypoxic conditions lead to tenocyte cell death and subsequent tendon degeneration. Hypoxia-induced apoptosis may be a critical step in the development of supraspinatus tendinosis. The long term goal of this research is to investigate the role of apoptosis in the development of tendinosis in vivo. Studies of tendon hypoxia will be conducted in tissue culture prior to animal experimentation to determine whether hypoxic conditions induce apoptosis in the tissue of interest. Cell death will be assessed using light microscopy. Apoptosis will be evaluated using TdT-mediated dUTP biotin nick end-labeling (TUNEL) assay. Therefore, the immediate objective of this project is to investigate the relationship between mechanical loading of tendon, hypoxia, and apoptosis of tendon fibroblasts in tissue culture. There are four specific aims of this pilot project: (1) to develop a tissue culture system for assessing the effect of mechanical loading and tissue hypoxia on tendon; (2) to determine if hypoxia induces apoptosis of rat supraspinatus tendon fibroblasts; (3) to determine if mechanical stress induces apoptosis in rat supraspinatus tendon fibroblasts; and (4) to determine if there is a synergistic effect of hypoxia and mechanical loading on apoptosis in rat supraspinatus tendon fibroblasts.